Dissociating loss of resources from loss of motivation during mental fatigue.

Few studies have looked at the relation between mental fatigue (MF) and motivation. In particular, it remains unclear whether the reduced performance sometimes observed following MF is caused by a progressive loss of cognitive resources or by a decrease in motivation to engage in mental activity. To tackle this question 18 healthy subjects were asked to participate in a 3,5 hour-long experiment where MF was induced by performing Sudoku tasks. The behavioural effects of MF were then measured at regular intervals, by means of a Simple Reaction Time task (SRT) and a Working Memory task (WMT) in which motivation was manipulated by different levels of monetary reward. Psychophysiological measurements (Pupillometry, Skin conductance, EEG and ECG) were also conducted during the execution of each task. Results in the SRT showed that the subjects were faster for higher incentives indicating the validity of the reward manipulation. We found that during the WMT, the participants’ performance got gradually worse across block repetition, reflecting the appearance of MF. Psychophysiological and behavioural measures concur in showing that this performance drop was not associated to decreased motivation and that the level of mental effort exerted during the task was maintained. This suggests that MF is mainly the consequence of a decline in mental resources.